IMO, there is a huge difference between growth slowing down (which I saw in my children) and fairly significant weight LOSS. Are they sure that 6 ounce weight loss is accurate (same scale, scale correct, naked both times, etc...)? That would be over 2% of body weight.
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You've already talked to her about increasing milk supply? Is she doing those things (feeding every 1-2 hours, night nursing...)?
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If there are no indications of low milk supply, I'm not sure formula will help. If she is going to start solids, make sure she does not start grain, fruits & veggies. They are very light density for calories. Avacodos are good. Another option is to add oils to simple grains. Or just oil if the baby will take it!
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When you say he appears happy and healthy, are the rest of his measurements increasing as expected? Head? Height? Is he rolling over? Meeting other developmental milestones. Is that an ongoing weight loss or is there a possiblity it was a one time innacuracy? (my DD "lost" more than that once but when we had her weighed at a different place she had, instead, gained about a point. Obvious scale error).